Been covered a few times, e.g.:http://ukbouldering.com/board/index.php/topic,3263.msg120207.html#msg120207http://ukbouldering.com/board/index.php?topic=12850.0
"something to brace the whole lot to stop it swinging"
cutting the ply wide enough to fit fairly snugly within the door frame and attach a rod/wooden baton beneath the FB which braces against the frame...
http://crusherholds.co.uk/fingerboard-mounting-device-2